The Collaborative International Dictionary of English v.0.48:

Post \Post\, n. [F. poste, LL. posta station, post (where horses
   were kept), properly, a fixed or set place, fem. fr. L.
   positus placed, p. p. of ponere. See Position, and cf.
   Post a pillar.]
   1. The place at which anything is stopped, placed, or fixed;
      a station. Specifically:
      (a) A station, or one of a series of stations, established
          for the refreshment and accommodation of travelers on
          some recognized route; as, a stage or railway post.
      (b) A military station; the place at which a soldier or a
          body of troops is stationed; also, the troops at such
          a station.
      (c) The piece of ground to which a sentinel's walk is
          limited.
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   2. A messenger who goes from station; an express; especially,
      one who is employed by the government to carry letters and
      parcels regularly from one place to another; a letter
      carrier; a postman.
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            In certain places there be always fresh posts, to
            carry that further which is brought unto them by the
            other.                                --Abp. Abbot.
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            I fear my Julia would not deign my lines,
            Receiving them from such a worthless post. --Shak.
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   3. An established conveyance for letters from one place or
      station to another; especially, the governmental system in
      any country for carrying and distributing letters and
      parcels; the post office; the mail; hence, the carriage by
      which the mail is transported.
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            I send you the fair copy of the poem on dullness,
            which I should not care to hazard by the common
            post.                                 --Pope.
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   4. Haste or speed, like that of a messenger or mail carrier.
      [Obs.] "In post he came." --Shak.
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   5. One who has charge of a station, especially of a postal
      station. [Obs.]
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            He held office of postmaster, or, as it was then
            called, post, for several years.      --Palfrey.
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   6. A station, office, or position of service, trust, or
      emolument; as, the post of duty; the post of danger.
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            The post of honor is a private station. --Addison.
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   7. A size of printing and writing paper. See the Table under
      Paper.
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   Post and pair, an old game at cards, in which each player a
      hand of three cards. --B. Jonson.

   Post bag, a mail bag.

   Post bill, a bill of letters mailed by a postmaster.

   Post chaise, or Post coach, a carriage usually with four
      wheels, for the conveyance of travelers who travel post.


   Post day, a day on which the mall arrives or departs.

   Post hackney, a hired post horse. --Sir H. Wotton.

   Post horn, a horn, or trumpet, carried and blown by a
      carrier of the public mail, or by a coachman.

   Post horse, a horse stationed, intended, or used for the
      post.

   Post hour, hour for posting letters. --Dickens.

   Post office.
      (a) An office under governmental superintendence, where
          letters, papers, and other mailable matter, are
          received and distributed; a place appointed for
          attending to all business connected with the mail.
      (b) The governmental system for forwarding mail matter.

   Postoffice order. See Money order, under Money.

   Post road, or Post route, a road or way over which the
      mail is carried.

   Post town.
      (a) A town in which post horses are kept.
      (b) A town in which a post office is established by law.


   To ride post, to ride, as a carrier of dispatches, from
      place to place; hence, to ride rapidly, with as little
      delay as possible.

   To travel post, to travel, as a post does, by relays of
      horses, or by keeping one carriage to which fresh horses
      are attached at each stopping place.
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WordNet (r) 3.0 (2006):

post horn
    n 1: wind instrument used by postilions of the 18th and 19th
         centuries